Add common, media_ctrl and v4l2_ctrl modules with CLI drivers and docs

Modules (src/modules/):
- common/error: App_Error struct with structured detail union, error codes,
  app_error_print(); designed for future upgrade to preprocessor-generated
  location codes
- media_ctrl: media device enumeration, topology query (entities/pads/links),
  link enable/disable via Media Controller API (/dev/media*)
- v4l2_ctrl: control enumeration (with menu item fetching), get/set via
  V4L2 ext controls API, device discovery (/dev/video*)

All modules use -std=c11 -D_GNU_SOURCE, build artifacts go to build/ only.
Kernel-version-dependent constants guarded with #ifdef + #warning.

CLI drivers (dev/cli/):
- media_ctrl_cli: list, info, topology, set-link subcommands
- v4l2_ctrl_cli: list, controls, get, set subcommands

Docs (docs/cli/):
- media_ctrl_cli.md and v4l2_ctrl_cli.md with usage, examples, and
  context within the video routing system

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/cli/media_ctrl_cli.md

@@ -0,0 +1,129 @@
+# media_ctrl_cli
+
+A development tool for exploring and configuring the Linux Media Controller pipeline. It wraps the `media_ctrl` module and provides a command-line interface to the Media Controller API (`/dev/media*`).
+
+This tool is the counterpart to `media-ctl` from the `v4l-utils` package. It covers the same ground but is built on our own `media_ctrl` translation unit.
+
+---
+
+## Build
+
+From the repository root:
+
+```sh
+make
+```
+
+Or from `dev/cli/` directly:
+
+```sh
+make media_ctrl_cli
+```
+
+The binary is placed in `dev/cli/`.
+
+---
+
+## Commands
+
+### `list`
+
+Enumerate all media devices present on the system.
+
+```sh
+./media_ctrl_cli list
+```
+
+Example output:
+
+```
+Media devices:
+  /dev/media0
+  /dev/media1
+```
+
+---
+
+### `info <device>`
+
+Show identification information for a media device.
+
+```sh
+./media_ctrl_cli info /dev/media0
+```
+
+Example output:
+
+```
+Device:          /dev/media0
+Driver:          unicam
+Model:           unicam
+Serial:
+Bus info:        platform:fe801000.csi
+Media version:   5.15.0
+Hardware rev:    0x00000000
+Driver version:  5.15.0
+```
+
+---
+
+### `topology <device>`
+
+Show the full pipeline topology: all entities, their pads, and all links between them. This is the main tool for understanding how a camera pipeline is structured.
+
+```sh
+./media_ctrl_cli topology /dev/media0
+```
+
+Example output on a Raspberry Pi with an IMX477 camera:
+
+```
+Device: unicam (/dev/media0)
+
+  Entity 1: imx477 4-001a
+    type:  v4l2-subdev (0x00020001)
+    flags: 0x00000000
+    pads:  1   links: 1
+      pad 0: source
+      link: entity 1 pad 0  ->  entity 2 pad 0  [enabled]
+
+  Entity 2: unicam-image
+    type:  devnode (0x00010001)
+    flags: 0x00000000
+    pads:  1   links: 1
+    device: 81:1
+      pad 0: sink
+      link: entity 1 pad 0  ->  entity 2 pad 0  [enabled]
+```
+
+Each entity shows:
+- **type** — the kernel entity function code and its human-readable name
+- **pads** — each pad with its direction (source or sink)
+- **links** — connections to other entity pads, with enabled/disabled state and whether the link is immutable
+
+---
+
+### `set-link <device> <src_entity>:<src_pad> <sink_entity>:<sink_pad> <0|1>`
+
+Enable or disable a link between two pads. Entity IDs and pad indices are the numeric values shown in the `topology` output.
+
+```sh
+# Enable a link
+./media_ctrl_cli set-link /dev/media0 1:0 2:0 1
+
+# Disable a link
+./media_ctrl_cli set-link /dev/media0 1:0 2:0 0
+```
+
+Immutable links cannot be changed and will return an error.
+
+---
+
+## Relationship to the Video Routing System
+
+`media_ctrl_cli` exercises the `media_ctrl` module, which is used in the video routing system for:
+
+- **Remote device enumeration** — a connecting node can query the media topology of a remote host to discover available cameras before opening any streams
+- **Pipeline configuration** — setting pad formats and enabling links is required before a V4L2 capture node can be used, particularly on platforms like the Raspberry Pi where the sensor and capture node are separate media entities
+
+See also: [`v4l2_ctrl_cli.md`](v4l2_ctrl_cli.md) for runtime camera parameter control.